Sunday, July 10, 2011

Soda Cans Beat Acoustic Diffraction Limit

I love experiments such as this. It is one of those experiments that uses ordinary objects, but produces astounding results, AND, gets published in PRL, of all places!

We have heard about optical metamaterial and how such materials can become left-handed and also defeat the diffraction limit. Well now, an acoustic metamaterial made up of soft drink cans (link open for free only for a limited time) has been made, and it can defeat the acoustic diffraction limit!

Lerosey and his colleagues took a similar approach to an experiment
they performed in 2007 and later described theoretically for
electromagnetic waves1,2.
The group generated audible sound from a ring of computer speakers
surrounding the acoustic 'lens': a seven-by-seven array of empty
soft-drink cans. Because air is free to move inside and around the cans,
they oscillate together like joined-up organ pipes, generating a
cacophony of resonance patterns. Crucially, many of the resonances
emanate from the can openings, which are much smaller than the
wavelength of the sound wave, and so have a similar nature to evanescent
waves.

To focus the sound, the trick is to capture these waves at any point
on the array. For this, Lerosey and his team used a method known as
time reversal: they recorded the sound above any one can in the
resonating array, and then played the recording backwards through the
speakers. Thanks to a quirk of wave physics, the resultant waveform
cancels out the resonance patterns everywhere — except above the chosen
can.

The work will be published in PRL (I'll put a reference to it here when it is published).